STUDIES FOR STUDENTS 



423 



Most of the foregoing statements need no explanation. No. 4 

 is due to the constant shifting of the wind and to the irregular 

 surfaces on which dust and sand are laid. In No. 5, the 30° dip 

 is the angle of the lee-sides of sand dunes, as read with the clinom- 

 eter, and is the angle of rest for sand. These are the characters 

 of eoHan sand, but, with the possible exception of 5, 7, g, and 10, 

 they also characterize loess. A typical section of dune sand is 

 shown in Fig. i. 



Fig. I. — A section of sand deposited by the wind near Cape Henry, Virginia 

 (photograph by Dr. T. W. Vaughan). 



CHARACTERS OF FLUVIAL DEPOSITS 



The conditions under which streams deposit are so various that 

 it is difficult and perhaps unwise to list any one set of characters 

 which will fit all fluvial materials. Swift streams deposit their 

 coarse loads at the bases of mountains under conditions quite 

 diffei:ent from those of larger and sluggish streams depositing on 

 their flood plains, and yet both distribute, both flow in one general 

 direction, both sort their materials at least roughly, and, after all, 

 the results differ chiefly in degree rather than in principle. Fluvial 

 deposits have the following characteristics: 



1. Relatively fine material, though some fans include large bowlders. 



2. High textural range (fans), and low textural range (some flood-plain 

 deposits) . 



3. Only roughly sorted. 



4. Lens and pocket structure. 



